This invention relates to a process for manufacturing an electronic circuit wherein an electronic device or component (part) such as a semiconductor integrated circuit (LSI) is connected to a circuit substrate, especially by soldering without using flux.
Soldering between a circuit substrate and a semiconductor integrated circuit (LSI) or the like has required that the surfaces of the metals in question to be joined should be kept clean and free from any material hindering wettability.
Plating has also required that the surface of the metal in question to be plated should be kept clean so that an oxide film or layer or the like does not exist thereon.
When an Au wire or Au ribbon or the like is to be bonded on the surface of the metal in question by an ultrasonic heat pressure technique, an oxide film on the surface of the metal in question is also troublesome and therefore the surface of the metal in question must be kept clean.
The materials hindering wettability of solder include oxides, chlorides, sulfides, carbonates, various types of organic compounds, etc. Particularly, the most serious obstacle in the processes such as soldering, plating or ultrasonic heat pressure bonding of an Au wire or Au ribbon is an oxide film present on the surface of the metal in question such as solder, nickel (Ni), nickel alloys (alloys of nickel with other substances).
An oxide film is usually chemically dissolved by flux and converted into a liquid compound. Thus, the surface of the metal in question and metal atoms of solder get an opportunity to directly collide with each other to form a metallic bonding state sharing an outer electron shell, so that the metal in question and solder can be alloyed. However, flux residues still remain on the surface and must be cleaned off.
The presence of an oxide film hinders plating. For example, an oxide film acts as an insulating (dielectric) film to prevent electric conduction therethrough necessary for electroplating, which is a typical example of plating, and thus hinders such electroplating.
Oxide film also hinders replacement plating by preventing the reaction between the surface of the metal in question and a plating liquid.
In these plating processes, oxide film should be removed by liquid treatment with hydrochloric acid or the like. However, residues still remain and are largely responsible for lowering the bonding reliability. Residues have been conventionally cleaned off with chlorofluorocarbons.
Recently, a technology has been developed to omit the step of cleaning off flux residues by using abietic acid (rosin) which leaves a lesser amount of residues as flux and a minor amount of adipic acid or the like. However, the bonding reliability is insufficient.
This technology is described in detail in "Alumit Technical Journal 19" (1992) and"Action Mechanism and Problems of Cleaning-Free Flux" by Kubota of Japan Industrial Technology Development Research Institute, Co.
A glazing method has also been proposed, which provides highly corrosion- and abrasion-resistant materials with very fine homogeneous texture or amorphous structure by irradiating metallic materials, steel materials, carbides or the like with laser beam. This glazing method is used for processing metallic materials to be subjected to high temperatures and high pressures such as materials for automobile turbines, and discussed in, for example, "Laser Processing, Second Series", by A. Kobayashi, pp. 164, published by Kaihatsusha.
A known method for removing oxide film on the surfaces of metals without using flux or hydrochloric acid employs argon sputtering.
Japanese Patent Application Laying Open (KOKAI or Unexamined Publication) No. 63-97382 discloses a method for providing a highly adhesive pinhole-free coating film by plating a surface of, a metallic workpiece, roughened by blasting with an alloy element, and thereafter melt processing the plated layer by irradiation with a laser beam.
Further, Japanese Patent Application Laying Open (KOKAI) 62-256961 discloses a method for preparing a surface-treated layer with good corrosion resistance and solderability by forming an anodized layer on the surface of a base material formed of aluminium or alloys thereof.